Largest sense?

The subject of this discourse will be imitation, as far as a painter is concerned in it. By imitation (explanation of "imagination" in context to the subject matter) I do not mean imitation in its largest sense,(not go to extremes with own imagination) but simply the following of other masters, and the advantage to be drawn from the study of their works.

In essence, the writer doesn't want you to use your own
imagination that could exceed the painter's imagination, which is explained by imagination and largest sense meaning: "going to extremes" with your own imagination
shouldn't be considered. You should figure out what the
painter imagined in his work and draw from the study of
their works - not your own.

I think the sentence is addressed to painters, not to viewers, of the art.

One way that art students learn is to copy works by other artists and imitate their style. When the writer says "not ... imitation in its largest sense" he means that the student should learn by imitation as a way to develop their own style, not simply imitate -- that would be forgery!

Thank you both.

By = with respect to.

Jane and Wy have nailed the author's meaning.

"imitation in it's largest sense" would usually mean "the most broad definition for imitation"...meaning, the definition which is most general and least specific, because largest would normally be referring to the scope of the meaning.

But, in this case, he meant "largest" to mean "most commonly used" which is misleading...the author's word choice there is not something you should emulate.